Anophthalmia and microphthalmia are defects in the development of the eye occurring spontaneously at a frequency of one to twelve percent in C57BL/6J mice. Several recently generated strains of C57BL/6J mice have an increased incidence of these defects including one line in which penetrance is complete, but dependent on the genetic background. These mice carry multiple copies of an agrin transgene fused to CFP. Agrin plays a critical role in development of the neuromuscular junction. Agrin is also expressed in the retina and hyaloid vessels although its function in these tissues is unknown. The first goal of this project is to characterize the expression of agrin-CFP in the developing eye as well as cataloging the types and initiation of various developmental defects. The second goal of this project, to map modifiers of anophthalmia and microphthalmia, takes advantage of the increased penetrance of these phenotypes in C57BL/6J mice. This work will increase understanding of how the eye develops and the role agrin plays in this process.